Fluoropolymers represent a class of compounds having noteworthy properties for a large number of applications, from painting or special coatings to sealing joints, optics, microelectronics, and membrane technology. Among these fluoropolymers, copolymers are particularly interesting due to their diversity, morphology, exceptional properties and versatility.
In particular, polymers based on VDF and TrFE have been proposed as ferroelectric materials which generate mechanical actuation induced by an external electric field. These polymers have been recognized for applications in a variety of transducers, actuators and sensors.
It has also been discovered that the use of a third monomer bearing a bulky substituent made it possible to disrupt the crystallization of the ferroelectric polymers based on VDF and TrFE, so as to give them properties of relaxor materials with significant electrostrictive effects.
Nonetheless, the introduction of this third monomer is generally reflected by a decrease in the melting point of the polymer and also a decrease in the mechanical strength thereof, measured by Young's modulus, which limits the possible uses thereof and constitutes an obstacle to the use thereof.
Document EP 0129244 describes P(VDF-TrFE-HFP) terpolymers, in which HFP denotes hexafluoropropylene, and also the manufacture thereof. More particularly, according to the document, the HFP monomer is first loaded into an autoclave, then a gaseous mixture of VDF and TrFE is added. The reaction is initiated by adding a radical polymerization initiator. This gaseous mixture of VDF and TrFE is then injected continuously to keep the pressure constant during the reaction.
Document FR 2889193 describes terpolymers of VDF and TrFE with a third monomer, especially CFE (1,1-chlorofluoroethylene), and the manufacture thereof. More particularly, according to the document, a gaseous mixture of VDF, TrFE and CFE is loaded into an autoclave with a polymerization initiator. A mixture of the three monomers is then injected continuously at constant pressure.
Document WO 2009/147030 describes P(VDF-TrFE) copolymers and also P(VDF-TrFE-CTFE) terpolymers, in which CTFE denotes chlorotrifluoroethylene, and also the manufacture thereof, which is carried out in a similar way to that which has been described above.
Document EP 0391421 also describes P(VDF-TrFE-CTFE) terpolymers and also the manufacture thereof. The document teaches preparing the terpolymers analogously to that which has been described above. In addition, in examples given by way of comparison, only an initial loading of monomers is provided, and the pressure in the reactor therefore decreases during the reaction. A terpolymer having a VDF/TrFE/CTFE molar ratio of 65/29/6 and having a melting point of 135° C. is obtained with this method.
Document U.S. Pat. No. 4,554,335 also describes P(VDF-TrFE-CTFE) terpolymers and also the manufacture thereof. According to the example provided, this manufacture is carried out by loading an autoclave with the three monomers according to a VDF/TrFE/CTFE molar ratio of 65/30/5, by heating to initiate polymerization and by continuing the reaction for 3 hours with a reduction in the pressure. The polymer obtained has a melting point of 140° C.
Document WO 2010/116105 describes a method for manufacturing P(VDF-TrFE-CTFE) or P(VDF-TrFE-CFE) terpolymers. This method proposes initial loading of only VDF and TrFE into an autoclave, initiation of the reaction by injecting initiator, then continuous injection of a mixture of the three monomers.
Other polymerization methods in the prior art rely on specific techniques of initiation chemistry. These are very difficult to implement on an industrial scale.
This is the case, for example, of the technique described in document U.S. Pat. No. 6,355,749, which relies on the use of organoborane molecules.
There is therefore a need to develop polymers based on VDF and TrFE having a relatively high melting point and a relatively high mechanical strength, and simultaneously properties of relaxor materials with significant electrostrictive effects.